So... do men REALLY think about sex every seven seconds?

You lose most body heat through your head. Chewing gum stays in your stomach for seven years. Lifting something heavy can give you a hernia.

Every day you hear or read things about your body and health that are simply not true. In many cases they've been scientifically discredited, yet still these medical myths endure - even doctors believe some of them!

In a new book, two leading paediatricians examine the evidence for many common health mantras - with some surprising revelations.

Over rated! Yes, men think about sex more often than women. But nearly half don't dwell on it even once a day

YOU USE ONLY 10PC OF YOUR BRAIN

Just imagine what you could do if you put your mind to it - after all, you're using only 10 per cent of your brain, right? People have believed this for more than 100 years. Unfortunately, all it means is that people have been wrong for over 100 years.

Different types of brain-imaging, including CT scans, MRI scans and even more detailed techniques show that no area of the brain is completely silent or inactive. Much more than 10 per cent of the brain is busy at work most of the time. So, as depressing as this may be, you are probably stuck with what you've got: you are, in fact, using 100 per cent of your brain.

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IT'S SAFE TO EAT FOOD OFF THE FLOOR

To most of us, it seems harmless enough to eat something you dropped
on the floor if it's been there no longer than five seconds (you might
not stick to it in a public loo, of course). But then you read the work
of scientists who specialise in food science and microbiology.

In a series of experiments, microbiologists looked at what
happens when the five-second rule comes up against Salmonella
typhimurium, a fairly common but nasty bacterium that can cause wretched diarrhoea and vomiting.

They
tested how well Salmonella survived on wood, tile and carpet, and they
tested how well it transferred from these surfaces to either sausage or
bread. First, they found that bacteria were still alive after four
weeks on dry wood, tile or carpet, and that enough of the bacteria
survived to be able to transfer to food.

So how long did it take for the bacteria to stick to the food?
The worst offender was sausage on tile. More than 99 per cent of the
bacterial cells transferred from the tile to the meat after just five
seconds of the sausage hitting the floor!

Transfer from
wood was a bit slower (5 to 68 per cent of the bacteria were
transferred) and transfer from carpet was not very successful (0.5 per
cent of the bacteria transferred, but it did so in under 5 seconds).

Basically, you just can't trust the five-second rule.

HAIR AND NAILS GROW AFTER YOU DIE

This
disturbing idea is probably the kind of thing you heard around a
campfire as a child. But it's just an optical illusion. After you die,
your body dries out - and as the skin dries out, it shrinks. When this
happens around the hair and nails, it makes them look longer or more
prominent. In order to keep growing, hair and nails require a complex
mixture of hormones that are not available to the body after death.

Midnight snack? Studies show eating late at night does not make you fat - it's how much you eat overall

URINE SHOULD BE CLEAR

If you listen to the 'experts', you probably think your urine should be pale yellow or almost clear, and if it's darker, you must be dehydrated and need more water - about six to eight glasses of water a day. Right?

This is really just a half-truth. It is true that your urine gets darker when there is less of it. But having yellow urine does not mean you are dehydrated. The colour depends on its osmolality, which is the technical term for how much stuff is dissolved in the liquid.

When there is more stuff dissolved in a given volume of urine, the urine is more concentrated, and therefore looks darker. However, the osmolality of normal urine can vary from person to person. Most of the time, normal urine from a healthy, well-hydrated person may be very yellow - if you are struggling to get it clear or pale yellow, you are probably worrying for nothing.

There is one caveat to dismissing this urine-colour business: if your doctor has recommended getting your urine lighter than normal because of a medical condition such as recurrent kidney stones, then do as they advise.

GUM STICKS IN YOUR GUT FOR YEARS

Chewing gum is made out of gum base, sweeteners, colouring and flavouring. The gum base is pretty indigestible; it is a mixture of elastomers, resins, fats, emulsifiers and waxes (sounds delicious). Most of the time, your stomach really can't break down the gum the way it would other foods. However, your digestive system has another way to deal with things you swallow.

After all, we eat lots of things that we can't fully digest. The gut just keeps moving them along until they make it all the way through the intestines and come out the other end. So gum usually ends up in your loo one to two days later. Even though gum is sticky, it is usually no match for the power of your gut.

The system can get gummed up though. Swallowing a huge wad of gum or swallowing many small pieces of gum in a short time can cause a blockage within the digestive system (most often in children, who have a smaller-diameter digestive tract than adults), but this is extremely rare.

NEVER WAKE SLEEPWALKERS

Many fear this will cause a heart attack, shock or brain damage. It's true that waking up a sleepwalker may confuse or frighten them, and they may even become violent in their confusion and try to hurt you - there have even been rare cases of sleepwalkers committing murder. So if you can guide the sleepwalker back to bed without waking them, that might be the best strategy!

But if you do wake them, they are not at any increased risk of heart attack or stroke.

GREEN MUCUS MEANS YOU HAVE A SINUS INFECTION

Many people believe that the colour of mucus matters. In particular, if the mucus has gone from clear to yellow or - the worst of the worst - green, then you must be getting a sinus infection, which means you need antibiotics straight away. Even doctors fall for the mucus colour myth.

However, scientific studies show that mucus colour does not predict whether you need antibiotics. In a study of children with green nasal discharge, 142 were assigned to take an antibiotic, a combination of a decongestant and antihistamine, a placebo or nothing at all.

Researchers found no difference, except that the kids taking the decongestant/antihistamine had more negative side-effects.

EATING LATE AT NIGHT MAKES YOU FAT

Health magazines are full of ways to change your eating habits to help you lose weight. One of the big suggestions is to not eat at night.

The rationale seems to be that if you eat later in the evening or before you go to bed, you won't have time to burn off those calories before you fall asleep. Or, perhaps, your metabolism slows down in the evening.

However, various studies have shown that it's not the time of day that matters, but how much you eat. For instance, one Swedish study of 86 obese men and 61 average-weight men found no differences in the timing of when they ate.

In a study of more than 2,500 patients of all weights, the time of day that they ate had nothing to do with whether or not they gained weight. Eating more than three meals per day has been shown to play a role in obesity. The key to weight loss or to maintaining a healthy weight is very simple: eat fewer calories than your body burns.

Trust us. As long as you do that, it doesn't matter what time of day you eat, you'll still lose weight.

FLU JABS CAN CAUSE THE FLU

THE idea that the flu jab gives you flu is just wrong. The flu jab uses a dead virus to protect you. Dead viruses cannot make you ill. Dead viruses cannot be resurrected to cause infections. They are dead.

We know someone out there will probably be arguing with this - maybe someone who thinks that the jab gave them flu in the past, but it didn't happen.

You probably just experienced a bad side-effect from the vaccine, such as low-grade fever and aches. This is not the flu. This is just the lousy part of getting a vaccination.

MEN THINK ABOUT SEX EVERY SEVEN SECONDS

It has become an accepted truth that men think about sex a lot. Depending on which version you've heard, it could be every few minutes or every seven seconds. On the face of it, this is simply ludicrous.

If we assume that the average male is awake for 16 hours a day, each man would have to think about sex more than 8,000 times a day. That's about as many times as a person breathes while awake.

The most comprehensive survey ever completed on sexuality, The Social Organisation of Sexuality: Sexual Practices in the United States found that 54 per cent of men reported
thinking about sex every day or several times a day, 43 per cent a few
times a month or a few times a week, and four per cent less than once a
month.

When asked the same question: 19 per cent of women reported thinking about sex every day or several times a day, 67 per cent a few times a month or a few times a week, and 14 per cent less than once a month.

Women do think about sex less than men, but the difference is not as great as men think.

A DOG'S MOUTH IS
CLEANER THAN A HUMAN'S

You've probably seen people give their dog wet slobbering kisses on the mouth - or maybe you are one of those people!

Many people defend this behaviour with the explanation that a dog's mouth is cleaner than a human's. This myth probably originated from the medical literature. Early studies showed that wounds caused by human bites were more likely to become infected than dog bites.

Later research shows this is only true of bites on the hand caused through 'clenched fist' interaction - in other words, if you get a bite on your hand because you punched someone in the mouth, this is more likely to get infected.

Humans and dogs have around the same amount of bacteria in their mouths.

LIFTING GIVES YOU A HERNIA

A hernia is technically an opening or a weakness in the muscle wall of the belly or groin. Lifting did not cause the hernia. However, when you lift something heavy, it increases the pressure in your abdomen.

If you already have a hernia, this type of increase in pressure can cause some of the contents of your belly to pop out through the hole. Lifting can cause this bulge to pop through or become more prominent, and so this makes your hernia more obvious.

Men with big feet: The idea that the size of a man's feet, hands or even
nose shed light on the size of his penis may actually be backed up by
science

MEN WITH BIG FEET...

Have you ever noticed a man with particularly big feet and wondered whether other parts of him were just as large? While some claim a man's penis size can be predicted by the size of his feet, others say it's the size of his hands or even his nose that really gives away the secret.

This idea of comparing body parts may have its roots in real science. The Hox gene in mammals plays a role in the development of the toes and fingers, as well as the penis or clitoris. Yet despite the similar genetic controls for these protuberances, men with big feet do not necessarily have bigger penises.

A study of 60 men in Canada suggested that there was a weak relationship between penile length and both body height and foot length.

However, a larger study of 104 men, done by two urologists, found that shoe size and penis size were not correlated. This was backed up by the Definitive Penis Size Survey, which looked at 3,100 men.

Studies also show no link between finger length and penis size. You can look at a guy's feet or hands all you want, but they won't tell you how he measures up.